Could my ex and father to my son be liable for breaking a visitation date when we planned a trip?

It was agreed in an email that my ex would have his summer time. He gets our son for two months in the summer. He chose May and July. I let him also know we had a trip planned in July to see my now husband's parents. I noticed I wasn't getting any itinerary in my email. So I decided to send my ex a message. He said he was waiting on a check. I asked him when he was planning to have our son visit him. He said the 17th which was after our planned trip. So now we may have to buy an extra ticket or pay for change fees. Would he be liable for anything since we made an agreement and told him we had a trip planned? Or should I just leave it alone?